Reproduction of sound records



March 1, 1938. K. SCHWARZ 2,109,754

REPRODUCTION OF SOUND RECORDS Filed July 2, 1935 vIHIIIIllllllllllllllllllm INVENTO ATTORNEY KAR- SCH R2 BY fig AAJLHA/ Patented Mar. 1, 1938 UNITED STATES PATENT OFFICE Karl Schwarz, Berlin,

Germany, assignor to Klangfilm G. m. b. 11., Berlin, Germany, a corporation of Germany Application July 2, 1935, Serial No. 29,438 In Germany October 9, 1934 2 Claims.

This invention relates to the reproduction of sound records whereon negative and positive half cycles are recorded on separate tracks, and has for its principal object the provision of an improved apparatus and method of operation whereby the audio currents reproduced from each of the tracks may be adjusted independently of one another so as to ensure that they are properly balanced in the input circuit of the amplifier through which they are supplied to a loudspeaker.

It has been proposed to employ two photo-electric cells connected in push-pull or phase opposition for reproducing photographic sound tracks in which the positive and the negative semicycles or alternations are recorded on separate sound tracks. In this arrangement the positive pole of a battery is connected to the anodes of the two cells and the negative pole of the battery is connected through one half of the primary winding of a transformer with the cathodes of the two photo-electric cells. From the secondary winding of the transformer may then be ob tained an alternating voltage whose positive and negative alternations consist of the two alternations comprised in the two separate tracks or recordings.

This invention relates to the elimination of a difliculty which is likely to be encountered if the two photo-electric cells do not have the same characteristic. In this case, recorded semicycles of exactly similar size and shape would be reproduced as an alternating voltage whose positive and negative halves difier from one another.

According to the invention, this difliculty is avoided by the provision of variable auxiliary sources of potential for each of the two photocell discharge paths, a distinct variable resistance or rheostat or other suitable means for equalizing the photocell outputs.

The invention will be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing,

Fig. 1 illustrates an arrangement wherein rectified and filtered potential is separately supplied to each of the output circuits of the push-pull reproducer,

Fig. 2 illustrates a modification wherein a common potential is applied to the output circuits through a resistor provided with taps which may be adjusted to balance the output currents, and

Figs. 3 and 4 illustrate further modifications.

Referring to Fig. 1, III denotes a film strip on which positive and negative half-cycle sound tracks are indicated at II and I2. These sound tracks modulate the illumination of the two photo-electric cells I3 and I4. The anodes of the two photocells are interconnected and united with the midpoint of the primary winding of a transformer I5. The cathodes are connected to the primary end terminals by way of a distinct source of potential which may be the potential drop of resistances I6 and I1, respectively. These potential drops are produced by an alternating voltage which is applied through a transformer I8 with regulable ratio of transformation, rectified by rectifiers I9 and smoothedor filtered by a condenser 20.

Another circuit scheme in which the auxiliary voltage sources for the two photo-electric cells are in part combined is illustrated in Fig. 2. The two photocells are again denoted by I3 and I4. Connected with the united anodes of the two cells is a resistance 2| through which a D. C. is fed by way of leads 22 and 23 in such a way that the left-hand end of the said resistance is at a positive potential in reference to its other (righthand) end. Between two contact sliders 24 and 25 and the two terminals 26 and 2'! of the photocell cathodes are connected the primary windings 28 and 29 of the transformer I5.

In the embodiment shown in Fig. 3, a common auxiliary voltage source is provided for the two photocells I3 and I4, the said source of voltage being formed by the fall of voltage across resistance 30. Each of the photocell circuits includes a separate regulating resistance 3| or 32 which is connected in parallel relation to the corresponding half of the primary winding of transformer I5. The middle of the primary winding is associated with the negative pole of the resistance 30, whereas the outer terminals of the primary winding 30 are brought by way of the resistance 33 and 34 and condensers 35 and 36 to the cathodes of the two photocells. The right-hand end of the resistance 30 is connected by way of two high-ohm resistances 31 and 38 with the photocell cathodes. In this arrangement, the two photocells are fed with the same D. C. voltage, i. e., the fall of potential caused across resistance 30, whereas the alternating potentials obtained from the different photocells may be independently varied by the adjustment of the resistances 3I and 32. If desired, the resistance 30 may be shunted by a condenser 39.

Fig. 4 shows a circuit arrangement which resembles the one shown in Fig. 2 in that the D. C. voltage for the two photocells is produced by different voltages which may be tapped on a resistance 2| by the aid of contacts 24 and 25. The primary windings of transformer [5, however, similarly as in Fig. 3, are connected with the photocell cathodes by way of condensers 35 and 36, while the various D. C. voltages are united, by Way of high-ohm resistances 37 and 38, with the photocell cathodes. In Fig. 4, the two condensers-MJ and. correspond to the condenser 39, Fig. 3.

I claim:

1. In a device for reproducing a sound record wherein the positive and negative half-cycles are recorded on separate tracks, the combination of a plurality of light responsive devices each arranged to have its illumination' modulated by a different one of said tracks, means for combining the outputs of said devicesrandimeans including the outputs of said devices, means including a resistor provided with adjustable taps for independently adjusting each of said outputs whereby "the said outputs may be balanced to avoid waveshape distortion, and condenser means connected in shunt to 'said resistor.

KARL SCHWARZ. 

